There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCD's), photodiode arrays, charge injection devices (CID's), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
CMOS imagers are well known. CMOS images are discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994); and are also disclosed in U.S. Pat. Nos. 6,140,630, 6,204,524, 6,310,366, 6,326,652, 6,333,205, and 6,326,868; assigned to Micron Technology, Inc., the entire disclosures of which are incorporated herein by reference.
Semiconductor imaging devices include an array of pixel cells, which converts light energy received, through an optical lens, into electrical signals. Each pixel cell contains a photosensor for converting a respective portion of a received image into an electrical signal. The electrical signals produced by the array of photosensors are processed to render a digital image.
The amount of charge generated by the photosensor corresponds to the intensity of light impinging on the photosensor. Accordingly, it is important that all of the light directed to the photosensor impinges on the photosensor rather than being reflected or refracted toward another photosensor as optical crosstalk.
For example, optical crosstalk may exist between neighboring photosensors in a pixel array. In an ideal imager, ideally, all the incident photons on top of a microlens are directed towards the photosensing element underneath that microlens. In reality, some of the photons get refracted and reach adjacent photosensors. This leads to undesirable optical crosstalk between neighboring pixels. This problem gets worse with scaled pixels and as the distance between the photosensor and the microlens increases. Increasing the number of interconnect metal layers typically increases this distance.
Optical crosstalk can bring about undesirable results in the images produced by the imaging device. The undesirable results can become more pronounced as the density of a pixel cell in imager arrays increases, and as pixel cell size correspondingly decreases. The shrinking pixel cell sizes make it increasingly difficult to properly focus incoming light on the photosensor of each pixel cell without accompanying optical crosstalk.
Optical crosstalk can cause a blurring or reduction in contrast in images produced by the imaging device. Optical crosstalk also degrades the spatial resolution, reduces overall sensitivity, causes color mixing, and leads to image noise after color correction. As noted above, image degradation can become more pronounced as pixel cell and device sizes are reduced. Furthermore, degradation caused by optical crosstalk is more conspicuous at longer wavelengths of light. Light having longer wavelengths penetrates more deeply into the silicon structure of a pixel cell, providing more opportunities for the light to be reflected or refracted away from its intended photosensor target.
One proposal to reduce optical crosstalk provides a continuous air-gap around the optical path to a photosensor. See Dun-Nian Yaung et al., Air-Gap Guard Ring for Pixel Sensitivity and Crosstalk Improvement in Deep Sub-micron CMOS Image Sensor, PROC. OF IEDM, 2003; see also T. H. Hsu et al., Light Guide for Pixel Cross Talk Improvement in Deep Submicron CMOS Image Sensor, IEEE ELECTRON DEVICE LETTERS, vol. 25, no. 1, 2004, at 22-24. FIG. 1 represents a cross sectional view of an imager showing two exemplary prior art techniques for dealing with optical crosstalk. The FIG. 1 imager has an air-gap guard ring 221 surrounding a photosensor optical path 223 existing between a micro-lens 240 and a photosensor 220. The air gap ring 221 is shown as being fabricated in the lower metallization layers M2 of an imager. The air gap provides a refraction index difference between the air gap (n2=1) and the surrounding dielectric layers (n1=1.4−1.6) and thus, the majority of incident light will be collected in the targeted pixel cell due to the total internal reflection in the air-gap/dielectric film interface. However, the presence of an air gap ring 221 is not ideally suited for solid state imagers. There are several reliability issues with the air gap such as its structural instability. Also, the color filter array (CFA) process, widely used in color imager fabrication, is known to have metallic and mobile ion contaminants that might easily diffuse through the air gaps and affect the devices and photosensor characteristics in the underlying pixel circuit.
Alternatively as also shown in FIG. 1, planar metal-shielding 225 provided in an upper metallization layer M4 has been used in an effort to reduce optical crosstalk, but these may degrade pixel sensitivity and/or are not suitable for use in zooming lens systems.
Another method of reducing optical crosstalk uses optical waveguides. Optical waveguides are structures used for spatially confining and directing light onto the intended target. For instance, optical waveguides can be used to reduce the detrimental affects associated with light shields such as light piping and light shadowing. Optical waveguides, however, are not widely used to focus light directly onto the photosensor in imaging devices. Moreover, currently employed optical waveguide structures, require additional processing steps, adding to the complexity and costs of imager fabrication.
Accordingly, there is a need and desire for an improved apparatus and method for reducing optical crosstalk in imaging devices. There is also a need to more effectively and accurately increase overall pixel sensitivity and provide improved optical crosstalk immunity without adding complexity to the manufacturing process and/or increasing fabrication costs.